1. Technical Field of the Invention
This invention relates to telecommunication systems. More particularly, and not by way of limitation, the present invention is directed to a system and method of estimating the position of a mobile terminal in a radio telecommunications network.
2. Description of Related Art
In modern radio telecommunication systems, the service area is divided into cells, each of which is served by a base station transceiver. As mobile terminals move about the service area of the system, they are handed off from one cell to another so that there is no lapse in service. In times of emergency, it would be extremely useful to police or other emergency crews responding to the situation to have precise position information for the mobile terminal. For this reason, the Federal Communications Commission (FCC) has mandated that cellular carriers be able to locate the position of a mobile terminal operating in their service areas to within a few hundred feet when the mobile terminal makes an emergency 911 (E911) call. The FCC mandate requires the position of the 911 caller to be computed and passed on to an emergency network operator.
One solution being considered by carriers operating Time Division Multiple Access (TDMA) networks is to locate mobile terminals using Received Signal Strength (RSS) measurements for signals transmitted by base stations and received at the mobile terminal. Of course, RSS-based position location, once implemented, can also be used for commercial location-based services.
A major problem with existing solutions for locating mobile terminals is the high cost and complexity required for their implementation into the existing cellular network infrastructure. Some solutions require extensive modifications to base stations or other components of the cellular telecommunication system. Others require extensive modifications to mobile terminals and are inadequate due to the large number of unmodified mobile terminals already in service.
Position location systems in general require multiple measurements either by the mobile terminal whose position is being determined, or by a set of fixed transceivers (base stations). The measurements are made on signals sent between the base stations and the mobile terminal. The measurements may include time of arrival, angle of arrival, or the signal strength of the received signal. Combinations of all three types may be used as well.
In general, the computation of the mobile terminal's position involves minimizing some error criterion given a set of functions that define the dependence of the measurements on the true position of the mobile terminal. For example, in the case of time of arrival measurements, this function is the difference between the time of the transmission and the time of reception of the radio signal which is simply the distance between the base station and the mobile terminal divided by the speed of light. While such functions exist for the case of RSS measurements (for example, the Okumura-Hata formula), they typically vary significantly with terrain. Also, when characterized accurately for each location (rather than in a statistical sense for a whole area), the functions generally do not have a convenient closed form mathematical expression. This is due to the large variance in RSS caused by fixed objects around the mobile terminal in each specific location.
In order to overcome the disadvantage of existing solutions, it would be advantageous to have a system and method of determining the position of a mobile terminal in a radio telecommunications network that can operate in an environment with high location-specific anomalies with a high degree of accuracy. The present invention provides such a system and method.